In television engineering, it is frequently necessary to delay a video signal by the duration of one or more lines, for example, for decoding a chrominance signal, in a PAL-decoder, in a SECAM-decoder, or in a video record player, during the reproduction of a color video signal recorded in trilinear sequence (German DAS No. 1,256,686).
If such a signal arrives from a recording device, the line duration of the signal can differ from the nominal value, due to the unavoidable speed fluctuations of the recording medium. A delay line with a constant delay time would then no longer delay the signal by the actual line duration, so that the delayed signals would no longer coincide, in time, with the undelayed signals. Thus, errors would appear, for example, in a reproduction circuit for a trilinear sequential signal.
It is known, from German Offenlegungschrift No. 2,261,301, to use an electronically timed storage to effect the delay by a line duration, and whose delay time depends on the frequency of the timing pulses controlling the storage. By phase comparison of the line sync pulses at the input and output of the storage, there is obtained a manipulated variable which controls the frequency of a timing pulse generator, and thus the effective delay time of the storage, in a manner such that the delay time of the storage is always equal to the actual duration of the line, even if such actual duration differs from the nominal duration value. The effective delay time then is always equal to the actual, possibly varying, line duration. This adaptation is possible with an electronically timed storage, because the delay time of the latter can be varied by varying the frequency of the timing pulse sequence contolling the storage. However, in the presently used ultrasonic delay lines, such an adaptation is not possible.
The electronically timed storages, mentioned above, and which are known, in practice, for example, as bucket chain circuits or as CCD-circuits (charge coupled device), have a limited amplitude modulation range. Thus, they limit the amplitude of the signal passing through the storage, and thus also the S/N, or signal-to-noise ratio.